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The chinchilla was used as an animal model in a study of the relation between noise exposure and hearing loss. Groups of chinchillas were exposed to noise while systematically varying the intensity, duration and scheduling of the exposures. Some of the animals were trained in a behavior response mode prior to exposure. Exposure to low frequency noise at 75 to 95 decibels sound pressure level caused permanent structural damage within the outer hair cells (OHCs). The damage initially began in the low frequency region of the cochlea but did not produce a permanent threshold shift for low frequency tones until the loss of apical OHCs exceeded 30%. Exposures of 9 days or longer caused additional damage in the high frequency region of the cochlea. These lesions were initially quite narrow but involved losses of inner hair cells (IHCs), OHCs, supporting cells and auditory nerve fibers. When the chinchillas were given 18 hours of rest between successive 6 hour exposures, loss of the apical OHCs was reduced somewhat from that associated with a continuous exposure of equal energy, and the development of a low frequency hearing loss was delayed. An 18 hour rest between exposures provided no protection from cell loss in the basal turn of the cochlea or from the development of permanent threshold shifts for high frequency tones. The authors suggest that these findings may explain why a 4 kilohertz notch is usually the initial manifestation of hearing damage in workers from noisy industries, regardless of the frequency content of their exposures.